The present invention relates to a communication system suitable for transmitting circuit switched and packet switched information over a common transmission link. In particular it relates to a system for transmitting circuit switched and packet switched information over a common time division multiplexed or time division multiple access transmission link in a fixed wireless access communication system or to a system in which packet switched information is transmitted across a channelised transmission link.
It is often desirable to transmit both circuit switched information and packet switched information from a first to a second location.
A circuit switched connection is characterised by the presence of a set-up phase, a connection phase during which a fixed amount of transmission capacity is assigned, and a tear down phase. Routing is determined during the set up phase and is constant throughout the connection. The advantage of circuit switching is that once a connection is established, the transmission capacity required for that connection is guaranteed. Thus, latency is both minimised and constant. The disadvantage is that the capacity is taken up by the connection even when no data is being sent. The predominant application for circuit switched connections is voice transmission (eg. telephony). Voice-band dial-up modems depend on circuit switched connections through the PSTN, and these provide a good example of the wasteful aspect of circuit-switching as the telephone call is maintained even when the data equipment has nothing to send.
A packet-switched data transfer is characterised by the division of the data payload to be transmitted into frames or packets, each of which carries a source and destination address. The packets are routed individually and so several data transfers can share a common transmission link using statistical multiplexing. Packet-switched networks can make use of buffering and flow control techniques (eg. Transmission Control Protocol) and can therefore adapt to changes in loading by increasing or decreasing the rate at which packets are transmitted from each source. A packet-switched approach will generally be more efficient than a circuit-switched approach in an application, such as Internet access, where the data sent is sufficiently bursty or intermittent, and where some variation in latency can be tolerated.
Separate transceiver facilities could be used between a first and a second location, one transceiver facility associated with a circuit switched transmission link and one transceiver facility associated with a packet switched transmission, as is described in International Patent Application WO 95/31060. However, this is a high cost option in view of the cost of duplicated transceiver facilities.
It is known to use a single transmission link, such as a time division multiplexed transmission link for transmitting both circuit switched information and packet switched information from a first to a second location. A problem exists with regard to the currently available arrangements for assigning time slots and/or channels on the transmission link to the two types of traffic.
One option is a fixed assignment wherein each type of traffic is permanently assigned to certain channels or circuits. This is undesirable as no provision is made for the different levels of traffic that exist at different times. For example, using a fixed assignment there could be unused or idle channels assigned to the circuit-switched information during times when all the channels assigned to the packet information are busy and there is a backlog of packet information waiting to be transmitted. Also, there could be times where new circuit-switched connections are blocked because all of the channels allocated to circuit switched traffic are in use during times when there is an under utilisation of the channels allocated to packet information.
This problem has been solved by the use of dynamic assignment mechanisms so that time slots in a transmission link are used efficiently. Such a system is described in EP 0236 102. In this system which operates in real time, preference in the use of time slots is normally given to circuit switched information, although packet switched information is assigned a minimum number of time slots so that there is never a total blockage of packet information in times of heavy circuit switched traffic. Dynamically operated control facilities are provided to allocate packet switched information to all time slots not currently allocated to a circuit switched connection and to all idle time slots currently allocated to a circuit switched connection.
The problem with the system proposed in EP 236 102 arises from the actions that are necessary whenever a new circuit-switched connection is required. The terminal control facilities must suspend packet data transfers, revise the channel allocations, and communicate the new channel assignments to a remote terminal, all before setting up the new circuit-switched connection (and restarting packet transfers). This may be acceptable in the point-to-point transmission links referred to in EP 236 102. However, in point-to-multi-point transmission links, it would be necessary to communicate the revised channel assignments to each of the remote terminals, possibly by means of unreliable transmission paths. The resulting delay in establishing a circuit connection would be significant in many applications. In the case of a TDMA fixed wireless access system, the technique described in EP 236 102 would result in an unacceptable delay before establishing a voice-band path between the telephone set and the local exchange. Alternatively, a means could be devised to set up the new circuit-switched connection without co-ordinating the packet assignments, but this would result in lost packets at a rate that would be unacceptable in most applications.
The same problem arises for the system decribed in GB2,301,992 in which channels in a TDMA transmission link are allocated dynamically to circuit switched information and the remaining time slots in the link are allocated dynamically to packet switched information. In this system packet switched information is preferentially allocated to idle time slots in a previously under utilised control channel in order to improve the efficiency with which both types of information are transmitted.
The object of the present invention is to provide a communication system suitable for transmitting circuit switched and packet switched information over a common transmission link in which the capacity of the link is improved without extending the set up time for circuit-switched connections.
According to a first aspect of the present invention there is provided a communication system including:
a transmission link having a number of channels; and
means for transmitting circuit switched and packet switched information over said transmission link;
wherein the system additionally includes a controller device for periodically pre-allocating channels to circuit switched traffic for a set time period dependent on one or more system parameters existing in a previous set time period.
Thus, at all times there are automatically pre-allocated channels for circuit switched traffic and the remainder of the link resource is available for use by packet switched traffic. This simplifies the process of allocating incoming traffic to channels and/or timeslots on the link as compared to the known methods for real time dynamic allocation of link resource. Additionally, this pre-allocation of channels is automatically optimised on a periodic basis in order to make good use of the link resource, as compared to static allocation, while maintaining the required grade of service to circuit traffic. Furthermore, if the peaks for circuit and packet switched traffic are offset in time then the capacity of the link will be increased by the use of the present invention. A further advantage is that the automatic allocation of channels by the controller device reduces the need for operator intervention and decreases the probability of the allocation of channels between circuit and packet switched traffic being incorrectly set.
Preferably, one of the system parameters is the number of circuit switched connections operating in a previous set time period. In this way the number of channels allocated to circuit switched connections can be made to follow the actual level of use of circuit switched connections by users of the system.
In a preferred embodiment the controller device comprises:
a monitoring unit for determining a first number equal to the number of operating circuit switched connections in a previous set time period;
a summing unit for forming a sum of the first number and an estimated margin;
means for allocating the summed number of channels to circuit switched connections for the next set time period; and
means for repeating the above steps after the next time period elapses.
By providing a margin of channels not currently used for operating circuit switched connections for anticipated future use by new circuit switched connections in the next time period, call blocking of circuit switched connections can be minimised while freeing up channels to packet switched traffic when circuit switched usage levels of the system are low. This will achieve the desired grade of service for circuit-switched traffic while optimising the grade of service for packet-switched traffic.
In a preferred embodiment a maximum number of the channels of the transmission link is set for circuit switched connections so that there are always some channels available on the transmission link for packet switched traffic. In this preferred embodiment the controller device may comprise:
a monitoring unit for determining a first number equal to the number of operating circuit switched connections in a previous set time period;
a summing unit for forming a sum of the first number and an estimated margin;
means for allocating a number of channels to circuit switched connections for the next set time period such that:
if the sum is greater than the pre-determined maximum number then the pre-determined maximum number of channels is allocated to circuit switched connections for the next time period; and
if the sum is less than the pre-determined maximum number then a number of channels equal to the sum is allocated to circuit switched connections for the next time period.
The present invention has particular advantage where the transmission link is a point-to multipoint link, for example a broadcast downlink or a multiple access uplink in a fixed wireless access communication system, because the periodic re-allocation of channels does not extend the set up time for circuit switched connections while enabling the efficient use of link resource responsive to the level of loading of the link.
In a preferred embodiment the estimated margin varies with the time of day in order to take into account the variability of loading of the transmission link due to the different patterns of link usage which occur at different times of the day. This means that the estimated margin can be smaller at least at some times of the day than in the embodiments where the estimated margin is constant. This further improves the efficiency of use of link resource according to the present invention.
The controller device will generally allocate the channels not allocated to circuit switched connections to packet switched traffic.
According to a second aspect of the present invention there is provided a controller device for allocating channels on a multi-channel transmission link to circuit switched traffic or packet switched traffic by periodically pre-allocating channels to circuit switched traffic for a set time period dependent on one or more transmission link parameters existing in a previous set time period. The controller device according to the second aspect of the invention has the preferred features of the controller device according to the first aspect of the present invention as set out above.
According to a third aspect of the present invention there is provided a base station in a fixed wireless access communication system comprising a controller device as described above for allocating channels on a multi-channel transmission link of the fixed wireless access communication system to circuit switched traffic or packet switched traffic.
According to a fourth aspect of the present invention there is provided a method of allocating channels on a multi-channel transmission link to circuit switched traffic or packet switched traffic comprising the step of periodically pre-allocating channels to circuit switched traffic for a set time period dependent on one or more transmission link parameters existing in a previous set time period. The method has the preferred features of the control system according to the first aspect of the present invention.
According to a fifth aspect of the present invention there is provided a computer program element comprising computer program code for making a computing device allocate channels on a multi-channel transmission link to circuit switched traffic or packet switched traffic by periodically pre-allocating channels to circuit switched traffic for a set time period dependent on one or more transmission link parameters existing in a previous set time period.